1. Field of the Invention
The present invention relates generally to the field of optical imaging, and more particularly to catadioptric optical systems used for microscopic imaging, inspection, and lithography applications.
2. Description of the Related Art
Many optical systems exist having the ability to inspect or image features on the surface of a specimen. Various applications employ microscopes for imaging purposes, including biology, metrology, semiconductor inspection, and other complex inspection applications wherein high resolution images of small areas and/or features are desired. Microscopes employ various imaging modes to enhance the appearance of desired features on the specimen. Imaging modes may include bright field, dark field, differential interference contrast, confocal, immersion, and others.
In many circumstances, a microscope capable of broad band imaging that also supports light energy in the ultraviolet (UV) wavelength can be highly advantageous. One example of this type of imaging is called optical coherence tomography (OCT). OCT uses a broadband microscope to provide high resolution cross-sectional imaging of biological tissues. In an OCT design, the coherence length of the light employed governs the longitudinal resolution of the image, and light coherence length is inversely proportional to the bandwidth of the light. In an imaging design that performs broad band imaging and supports light energy in the UV wavelength range can provide improved lateral and longitudinal resolutions of the resultant image.
The imaging objective is one particular optical component that enables the microscope to perform broad band imaging while simultaneously supporting light wavelengths below 400 nm. Certain UV objectives are capable of transmitting light down to a wavelength of 340 nm, but these objectives do not provide accurate imaging performance for light wavelengths below 400 nm. These types of objectives are mainly used for fluorescence, where wavelengths from 340 nm through the visible light spectrum excite fluorescence components in marker dyes. Fluorescence type imaging is typically employed in the visible light spectrum, so imaging performance in the visible light spectrum is the specific type of performance required.
Few objectives are available that provide broad band performance at wavelengths below 400 nm. Of the available objectives, none can be used in a standard microscope system. They are either too large, have insufficient numerical aperture (NA), or have an insufficient field size.
With respect to NA, the NA of an objective represents the objective's ability to collect light and resolve fine specimen detail at a fixed object distance. Numerical aperture is measured as the sine of the vertex angle of the largest cone of meridional rays that can enter or leave the optical system or element, multiplied by the refractive index of the medium in which the vertex of the cone is located. A large numerical aperture provides distinct advantages during inspection, not the least of which is an ability to resolve smaller features of the specimen. Also, high NAs collect a larger scattering angle, thereby improving performance in darkfield environments.
Two patents that disclose broad band, highly UV corrected, high numerical aperture (NA) catadioptric systems are U.S. Pat. No. 5,717,518 to Shafer et al. and U.S. Pat. No. 6,483,638 to Shafer et al. A representative illustration of a catadioptric design 100 in accordance with the teachings of the '518 patent is presented in FIG. 1, which is similar to FIG. 1 of the '518 patent. A representative illustration of a catadioptric design 200 in accordance with the teachings of the '638 patent is presented in FIG. 2, which has similarities to FIG. 4 of the '638 patent.
U.S. Pat. No. 5,717,518 to Shafer et al. discloses an imaging design capable of high NA, ultra broadband UV imaging. The high NA (up to approximately 0.9) system can be used for broadband bright field and multiple wavelength dark-field imaging. Certain issues exist with designs similar to that presented in FIG. 1. First, the field lens group may need to be physically located within a central hole in the large curved catadioptric element, which can make manufacturing difficult and expensive. Second, the field lens elements in such a design may require at least one glued interface. In the presence of wavelengths less then 365 nm, reliable glues that can withstand light intensity levels at an internal focus are generally unavailable. Third, the lens elements in such a design may be located very close to a field plane, thereby requiring a high degree of, or nearly perfect, surface quality and bulk material quality to prevent image degradation. Fourth, element diameters are typically larger than a standard microscope objective, especially for the catadioptric group. Large diameter elements frequently make integration into an inspection system difficult and can increase manufacturing costs.
The design of FIG. 2 is generally capable of high NA, ultra broadband UV imaging. The design is a high NA (up to approximately 0.9) imaging system that can be used for broadband bright field and multiple wavelength dark-field imaging and can use a varifocal tube lens to provide a large range of magnifications. The FIG. 2 design introduces very tight tolerances in the field lens group, due in part to increased on-axis spherical aberration produced by the catadioptric group. This on-axis spherical aberration must be corrected by the following refractive lens elements. The design of FIG. 2 is relatively large, thereby generally requiring complicated optomechanical mounting of elements, especially in the catadioptric group.
With regard to the high NA designs of FIG. 1 and FIG. 2, microscopes place several significant constraints on the objectives employed. Distance from the specimen to the mounting flange is typically in the range of 45 mm, while most objective turrets limit the objective diameter to 40 mm. Threads used to screw the objective into the microscope turret are typically either 20 mm, 25 mm, or 32 mm. Many microscope objectives employ all refractive design approaches, where all components are refractive elements. If an all refractive design is employed, the wavelength range used is limited by the available glass materials.
Other optical arrangements have been developed to perform specimen inspection in the microscopy field, but each arrangement tends to have certain specific drawbacks and limitations. Generally, in a high precision inspection environment, an objective with a short central wavelength provides advantages over those with long central wavelengths. Shorter wavelengths can enable higher optical resolution and improved defect detection, and can facilitate improved defect isolation on upper layers of multi-layer specimens, such as semiconductor wafers. Shorter wavelengths can provide improved defect characterization. An objective that can cover as large a wavelength range as possible may also be desirable, particularly when using an arc lamp as an illumination source. An all refractive objective design is difficult in this wavelength range because few glass materials having high transmission are effective for chromatic correction. A small bandwidth may not be desirable for inspection applications due to limitation of available light power and increased interference from thin films on the surface being inspected.
Small objectives are also desirable, as small objectives can be used in combination with standard microscope objectives and physically fit within standard microscope turrets. The standard objective flange-to-object distance is in the range of 45 mm. The available catadioptric objectives frequently cannot satisfy this requirement, so special microscope systems can be employed having an objective flange-to-object distance in excess of 60 mm and having lens diameters greater than 60 mm. Certain of these smaller objectives have NAs limited to 0.75 and field sizes limited to 0.12 mm with a bandwidth less than 10 nm. Such designs typically use a Schwartzchild approach with lenses added within the catadioptric group in an effort to improve performance. Working distances are typically greater than 8 mm. Using a shorter working distance with this type of design approach can somewhat reduce the objective diameter at the cost of increasing central obscuration, significantly degrading objective performance.
An objective having low intrinsic aberrations is also desirable, as is an objective that is largely self-corrected for both monochromatic and chromatic aberrations. A self corrected objective will have looser alignment tolerances with other self corrected imaging optics. An objective with loose manufacturing tolerances, such as lens centering tolerances, may be particularly beneficial. Further, reducing incidence angles on lens surfaces can have a large effect on optical coating performance and manufacturing. In general, lower angles of incidence on lens surfaces also produce looser manufacturing tolerances.
It would be beneficial to provide a system for use in microscopy that overcomes the foregoing drawbacks present in previously known systems and provide an optical inspection system design having improved functionality over devices exhibiting those negative aspects described herein.